The basic component of a magnetic tunnel junction is a sandwich of two thin ferromagnetic and/or ferrimagnetic layers separated by a very thin insulating layer through which electrons can tunnel. The tunneling current is typically higher when the magnetic moments of the ferromagnetic (F) layers are parallel and lower when the magnetic moments of the two ferromagnetic layers are anti-parallel. The change in conductance for these two magnetic states can be described as a magneto-resistance. Here the tunneling magnetoresistance (TMR) of the MTJ is defined as (RAP−RP)/RP where RP and RAP are the resistance of the MTJ for parallel and anti-parallel alignment of the ferromagnetic layers, respectively. MTJ devices have been proposed as memory cells for nonvolatile solid state memory and as external magnetic field sensors, such as TMR read sensors for heads for magnetic recording systems. For a memory cell application, one of the ferromagnetic layers in the MTJ is the reference layer and has its magnetic moment fixed or pinned, so that its magnetic moment is unaffected by the presence of the magnetic fields applied to the device during its operation. The other ferromagnetic layer in the sandwich is the storage layer, whose moment responds to magnetic fields applied during operation of the device. In the quiescent state, in the absence of any applied magnetic field within the memory cell, the storage layer magnetic moment is designed to be either parallel (P) or anti-parallel (AP) to the magnetic moment of the reference ferromagnetic layer. For a TMR field sensor for read head applications, the reference ferromagnetic layer has its magnetic moment fixed or pinned so as to be generally perpendicular to the magnetic moment of the free or sensing ferromagnetic layer in the absence of an external magnetic field. The use of an MTJ device as a memory cell in an MRAM array is described in U.S. Pat. No. 5,640,343. The use of an MTJ device as a MR read head has been described in U.S. Pat. Nos. 5,390,061; 5,650,958; 5,729,410 and 5,764,567.
FIG. 1A illustrates a cross-section of a conventional prior-art MTJ device. The MTJ 100 includes a bottom “fixed” or “reference” ferromagnetic (F) layer 15, an insulating tunnel barrier layer 24, and a top “free” or “storage” ferromagnetic layer 34. The MTJ 100 has bottom and top electrical leads 12 and 36, respectively, with the bottom lead being formed on a suitable substrate 11, such as a silicon oxide layer. The ferromagnetic layer 15 is called the fixed (or reference) layer because its magnetic moment is prevented from rotating in the presence of an applied magnetic field in the desired range of interest for the MTJ device, e.g., the magnetic field caused by the write current applied to the memory cell from the read/write circuitry of the MRAM. The magnetic moment of the ferromagnetic layer 15, whose direction is indicated by the arrow 90 in FIG. 1A, can be fixed by forming it from a high coercivity magnetic material or by exchange coupling it to an antiferromagnetic layer 16. The magnetic moment of the free ferromagnetic layer 34 is not fixed, and is thus free to rotate in the presence of an applied magnetic field in the range of interest. In the absence of an applied magnetic field, the moments of the ferromagnetic layers 15 and 34 are aligned generally parallel (or anti-parallel) in an MTJ memory cell (as indicated by the double-headed arrow 80 in FIG. 1A) and generally perpendicular in a MTJ magnetoresistive read head. The relative orientation of the magnetic moments of the ferromagnetic layers 15, 34 affects the tunneling current and thus the electrical resistance of the MTJ device. The bottom lead 12, the antiferromagnetic layer 16, and the fixed ferromagnetic layer 15 together may be regarded as constituting the lower electrode 10.
The basic concept of a magnetic tunnel junction was first realized in 1975 (M. Julliére, “Tunneling between ferromagnetic films”, Phys. Lett. 54A, 225 (1975)), although the TMR was very small and observed only at low temperatures and for very small bias voltages. In 1995 significant TMR effects of about 10% were obtained at room temperature in MTJs with Al2O3 tunnel barriers by two different groups (J. S. Moodera et al., “Large magnetoresistance at room temperature in ferromagnetic thin film tunnel junctions”, Phys. Rev. Lett. 74, 3273 (1995); and T. Miyazaki and N. Tezuka, “Giant magnetic tunneling effect in Fe/Al2O3/Fe junction”, J. Magn. Magn. Mat. 139, L231 (1995)). Subsequently, S. S. P. Parkin et al. (“Exchange-biased Magnetic Tunnel Junctions and Application to Non-Volatile Magnetic Random Access Memory”, J. Appl. Phys. 85, 5828 (1999)) obtained effects as large as about 48-50% by optimizing the growth of the Al2O3 tunnel barrier, by optimizing the interface between the Al2O3 tunnel barrier and the ferromagnetic electrodes, and by carefully controlling the magnetic orientation of the ferromagnetic moments using concepts of magnetic engineering, in particular, exchange bias (see U.S. Pat. No. 5,650,958 titled “Magnetic tunnel junctions with controlled magnetic response” to Gallagher et al.) and an anti-parallel coupled pinned ferromagnetic layer (see U.S. Pat. No. 5,841,692 titled “Magnetic tunnel junction device with antiferromagnetically coupled pinned layer” to Gallagher et al.).
The magnetoresistance of MTJs using aluminum oxide tunneling barriers is limited to about 50% at room temperature (S. S. P. Parkin et al., “Exchange-biased Magnetic Tunnel Junctions and Application to Non-Volatile Magnetic Random Access Memory”, J. Appl. Phys. 85, 5828 (1999); X.-F. Han et al., “Fabrication of high-magnetoresistance tunnel junctions using CO75Fe25 ferromagnetic electrodes”, Appl. Phys. Lett. 77, 283 (2000)), although there have been reports of TMR values of up to about 58% at room temperature (M. Tsunoda et al., “60% magnetoresistance at room temperature in Co—Fe/Al—O/Co—Fe tunnel junctions oxidized with Kr—O2 plasma”, Appl. Phys. Lett. 80, 3135 (2002)).
For applications of magnetic tunnel junctions for either magnetic recording heads or for non-volatile magnetic memory storage cells, high TMR values are needed for improving the performance of these devices. The speed of operation of the recording head or memory is related to the signal to noise ratio (SNR) provided by the MTJ—higher TMR values will lead to higher SNR values for otherwise the same resistance. Moreover, for memory applications, the larger the TMR, the greater is the variation in resistance of the MTJs from device to device which can be tolerated. Since the resistance of an MTJ depends exponentially on the thickness of the tunneling barrier, small variations in thickness can give rise to large changes in the resistance of the MTJ. Thus high TMR values can be used to mitigate inevitable variations in tunnel barrier thickness from device to device. The resistance of an MTJ device increases inversely with the area of the device. As the density of memory devices increases in the future, the thickness of the tunnel barrier will have to be reduced (for otherwise the same tunnel barrier material) to maintain an optimal resistance of the MTJ memory cell for matching to electronic circuits. Thus a given variation in thickness of the tunnel barrier (introduced by whatever process is used to fabricate the MTJ) will become an increasingly larger proportion of the reduced tunnel barrier thickness and so will likely give rise to larger variations in the resistance of the MTJ device.
U.S. patent application Ser. No. 10/824,835 to Parkin titled “MgO tunnel barriers and method of formation” (filed Apr. 14, 2004), which is hereby incorporated by reference, discloses a method of forming a tunnel barrier comprised of magnesium oxide (MgO) with which magnetic tunnel junctions can be deposited which exhibit tunneling magnetoresistance values of more than 100% at low bias. The tunnel barrier is formed by first depositing a thin layer of Mg using, for example, magnetron or ion beam sputter deposition followed by a layer of Mg deposited in the presence of oxygen. In addition, Parkin discloses methods of forming highly oriented crystalline MgO tunnel barriers by forming the MgO barrier on a ferromagnetic electrode comprised of a Co—Fe alloy, which is bcc and (100) textured. The CoFe electrode is formed on a (100) oriented antiferromagnetic layer of fcc IrMn which itself is grown highly oriented by forming this layer on suitable underlayers, for example, a combination of a TaN layer followed by a Ta layer.
Useful MTJ devices for magnetic recording read heads or for MRAM memory cells will be of sub-micron dimensions. This leads to very large self-demagnetizing for devices which are not circular in cross-section and very large magnetostatic coupling fields between ferromagnetic layers in the same device. For example, in the conventional device shown in FIG. 1A, there will be a very large interaction between the pinned ferromagnetic layer 15 and the free or storage ferromagnetic layer 34 because of magnetic poles formed at the edges of the device 100. These coupling fields are so large as to make such devices typically unworkable because the direction of the magnetic moment of the storage layer 34, indicated by the arrow 80 in FIG. 1A, will preferentially be oriented antiparallel to that of the direction of the magnetic moment of the fixed ferromagnetic layer 15, indicated by the arrow 90 in FIG. 1A. One method to solve this problem was first proposed by Parkin and Heim with reference to metallic spin-valve giant magnetoresistance sensors in U.S. Pat. No. 5,465,185, wherein the reference ferromagnetic layer 15 is replaced by a sandwich of two ferromagnetic layers 18 and 19 antiferromagnetically coupled through a metallic spacer layer 17 as shown by the MTJ 100′ of FIG. 1B. The lower electrode is now given by the reference numeral 10′, and the magnetic orientation of the layers 18 and 19 is given by the arrows 90′ and 95, respectively. Parkin showed that the spacer layer can be comprised of a wide variety of non-magnetic metals chosen from the groups of the 3d, 4d, and 5d transition metals as well as the noble metals, Cu, Au and Ag such that the layers 18 and 19 are indirectly exchange coupled through the metallic spacer layer 17 (S. S. P. Parkin et al. “Oscillations in Exchange Coupling and Magnetoresistance in Metallic Superlattice Structures: Co/Ru, Co/Cr and Fe/Cr”, Phys. Rev. Lett. 64, 2304 (1990) and S. S. P. Parkin, “Systematic Variation of Strength and Oscillation Period of Indirect Magnetic Exchange Coupling through the 3d, 4d and 5d Transition Metals”, Phys. Rev. Lett. 67, 3598 (1991)). For certain thicknesses of the spacer layer 17, the magnetic moments of the ferromagnetic layers 18 and 19 are antiferromagnetically coupled to one another so that the net magnetic moment of the sandwich can be chosen to be arbitrarily small. Consequently, the demagnetization field from the edges of the layer 18 is reduced by the opposite demagnetizing field arising from the poles at the edges of the layer 19. The net demagnetizing field can be zero by proper choice of the thicknesses and the magnetic material forming layers 18 and 19. In particular Parkin (S. S. P. Parkin et al. “Oscillations in Exchange Coupling and Magnetoresistance in Metallic Superlattice Structures: Co/Ru, Co/Cr and Fe/Cr”, Phys. Rev. Lett. 64, 2304 (1990)) showed that Ru is a highly preferred antiferromagnetic coupling layer because of the large antiferromagnetic (AF) coupling strength exhibited by very thin layers of Ru and because Ru displays large AF coupling for a wide range of ferromagnetic materials. Moreover, structures using Ru antiferromagnetic coupling layers also display high thermal stability. For these reasons the synthetic antiferromagnetic structure formed from the combination of ferromagnetic layers 18 and 19 separated by a thin Ru layer 17 has become the de facto structure of choice for magnetic recording read heads based on giant magnetoresistance as well as for magnetic tunnel junction memory cells based on spin dependent tunneling using amorphous alumina tunnel barriers. The use of synthetic antiferromagnetic reference layers using Ru antiferromagnetic coupling layers for MTJ sensor and memory applications is described in U.S. Pat. No. 5,841,692 titled “Magnetic tunnel junction device with antiferromagnetically coupled pinned layer” to Gallagher et al.
However, MTJs with MgO tunnel barriers and synthetic antiferromagnetic reference layers using Ru antiferromagnetic coupling layers do not exhibit the high tunneling magnetoresistance values exhibited by similar MTJs without the synthetic antiferromagnetic reference layer. MTJs without a synthetic antiferromagnetic reference layer suffer from the presence of unwanted coupling fields, as discussed above, since the storage magnetic layer moment has a tendency to be aligned anti-parallel to the reference layer magnetic moment. Alternative materials and structures are needed for the preparation of MTJ devices with high tunneling magnetoresistance.
In a cross-point MRAM the MTJ devices are switched by the application of two magnetic fields applied along two orthogonal directions, the easy and hard axes of the MTJ device. Typically the magnetic easy and hard anisotropy axes are defined by the shape of the MTJ element through the self demagnetizing fields. The MRAM array will contain a series of MTJ elements arranged along a series of word and bit lines, typically arranged orthogonal to one another. The magnetic switching fields are realized by passing currents along the word and bit lines. All the cells along a particular word or bit line will be subject to the same word or bit line field. Thus the width of the distribution of switching fields for the selected MTJs (those subject to the vector sum of the bit and word line fields) must be sufficiently narrow that it does not overlap the distribution of switching fields for the half-selected devices. The moment within the MTJ devices lies along a particular direction, the easy axis. The orthogonal direction is the magnetic “hard” axis.
One of the most challenging problems for the successful development of MTJ memory storage cells is to obtain sufficiently uniform switching fields for a large array of MTJ cells. The switching field will depend on the detailed structure, size and shape of the individual MTJ cells. In particular, the switching characteristics of the storage layer will be influenced by the detailed crystallographic structure of this layer. The storage layer will be formed from a multilayered stack of thin layers comprising both ferromagnetic and non-ferromagnetic materials. Typically the storage layer will be polycrystalline, i.e., comprised of grains of crystalline material having a lateral extent in the range of 100 to 500 angstroms. The ferromagnetic materials will likely exhibit magnetocrystalline anisotropy whereby the magnetization within a grain will prefer to lie along a particular crystallographic direction within the grain. Since the crystalline orientation of the grains which comprise the ferromagnetic layer will vary randomly from grain to grain, the preferred orientation of the magnetic moment will also vary in direction from grain to grain—this direction can be in the plane of the film or could be at an angle out of or into the plane of the film.
For grains which are strongly exchange coupled, the magnetic moment of the ferromagnetic layer will exhibit a preferred direction determined by averaging over the contributions from the various grains. For a MTJ device comprised of a great number of grains, the contribution of any individual grain is small. However, as the size of the MTJ storage cell is shrunk, the number of crystalline grains will also be reduced, assuming the grain size is unchanged, so that eventually the properties of individual cells may depend on the properties of a small number of grains. This would lead to variations in magnetic switching field from device to device, which can reduce the write margin. One means of mitigating this problem might be to reduce the size of the crystalline grains within the MTJ storage cell storage layer or, alternatively, to eliminate the grains altogether by forming the storage layer from amorphous ferromagnetic (or ferrimagnetic) materials as described in U.S. patent application Ser. No. 10/720,962 to Parkin and Samant titled “Magnetic tunnel junction with improved tunneling magneto-resistance” (filed Nov. 24, 2003), which is hereby incorporated by reference.
While forming the storage layer within the MTJ memory cell from amorphous magnetic materials would have the advantage of reducing the variation in magnetic switching fields, it is in general desirable for the MTJ to exhibit the largest possible tunneling magnetoresistance, so as to allow for unavoidable cell-to-cell variations in resistance of the MTJ devices within an MRAM array of cells. The methods of forming MTJs with high tunneling magnetoresistance values described in U.S. patent application Ser. No. 10/824,835 to Parkin titled “MgO tunnel barriers and method of formation” (filed Apr. 14, 2004) lead to MTJs which are polycrystalline and highly textured. The high tunneling magnetoresistance requires that the MgO tunnel barrier be simple cubic and preferably (100) textured and surrounded on both sides by ferromagnetic layers which are bcc and preferably (100) textured. The highly textured nature of the MTJ cell means that the easy axis of magnetization will lie preferentially within the plane of the MTJ storage layer. This means that the variation in magnetocrystalline anisotropy field will be typically larger from device to device than would be the case if the storage layer was poorly textured, so that the easy axis of magnetization preferentially would not be within the plane of the storage layer. Therefore, the highly textured structure of MTJs with MgO tunnel barriers which exhibit high tunneling magnetoresistance is likely to lead to large variations in magnetic switching fields from device to device. Since the high tunneling magnetoresistance properties of the MTJs depends on the highly textured crystallographic orientation of the MTJ cell, the use of amorphous ferromagnetic layers, which would likely give rise to smaller variations in magnetic switching fields, would appear to be incompatible with the structural requirements of high TMR MTJ devices.
What is needed is a method of forming MTJs with high tunneling magnetoresistance with improved magnetic switching characteristics.